Hazelnut caramel cake

Method

1. Heat the oven to 180°C/160°C fan/gas 4. In a large mixing bowl, beat the butter and sugar using an electric mixer until pale and fluffy. Beat in the eggs and yolks in 2 batches, followed by the soured cream and vanilla. Fold in the flour and baking powder with a large pinch of salt using a metal spoon in a figure-of-eight motion. Once the mixture is smooth, divide it evenly between 2 x 20cm greased and lined sandwich tins. Bake for 30-35 minutes until risen, golden and springy to the touch. Remove from the oven, cool in the tins for 10 minutes, then turn out onto wire racks to cool completely. 
2. For the caramel icing, put the sugar in a large, spotlessly clean frying pan and heat very gently, without stirring, until the sugar melts and turns into brick red caramel. Remove the pan from the heat, then stir in the butter, being careful as it may spit. Once the butter has melted, stir in the cream, then return to the heat and bring to a gentle simmer. Stir for 3-4 minutes until thickened. 
3. Transfer the caramel to a clean bowl and cool completely, then stir in the hazelnuts. The quickest way to cool the caramel is to put the bowl in a larger bowl filled with ice cubes and water. Stir it until it thickens, becomes matte and much paler, and holds its shape when the spatula is removed. Alternatively, you can leave it to cool, then chill until quite firm (this takes much longer).
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4. Once the caramel icing is very thick and you’ve stirred in the nuts, spread a third on top of one of the sponges. Top with the second sponge, then spread the remaining caramel all over the cake, using a palette knife to smooth the top and sides, and gradually adding more as it hardens. Scatter sea salt flakes over the top, then serve or store in an airtight container.